Fig. 2
Schematic of estimated ranges (shaded areas) for BS:PM10 and BS:PM2.5 ratios in UK urban areas between 1970 and 2000 based on data from the observational studies discussed in the text. The shaded areas do not represent formal statistical confidence intervals. If the average size of the ‘black’ (i.e. optically absorbing) particles in the particle mixture was decreasing through this time period then the reduction in the proportion of black particles in the PM mixture may be slightly greater than the reduction in the numerical ratios between BS and PMx illustrated in this figure (see text for more detail). The middle shaded area represents the overlap between the 2 sets of estimates